Filter apparatus and method for the production of sterile skimmed milk

ABSTRACT

The present invention relates to a method for the production of commercially sterile skimmed milk. After separation, the skimmed milk fraction is treated by microfiltration in one or more steps. The microfiltration gives a permeate flow and a retentate flow, in which all microorganisms of a given size have effectively been separated from the permeate flow. After the microfiltration, the permeate flow is heat treated at a temperature of 72-134°C. during a requisite period of time. In order to obtain a sterile skimmed milk which is as good as free of destroyed microorganisms, the retentate flow is not employed.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/380,833, filed Dec. 2, 1999, now abandoned.

TECHNICAL FIELD

The present invention relates to a method for the production of sterileskimmed milk.

BACKGROUND ART

Sterile milk, or commercially sterile milk may be defined as a productwhich is free of microorganisms which can grow under the prevailingconditions. A sterile milk, packed under aseptic conditions in asepticpackages, enjoys the advantage of being able to be distributed andstored at room temperature for a lengthy period of time.

In order to produce such a commercially sterile milk, the commonestmethod hitherto has been UHT treatment (Ultra High Temperature), i.e.treatment of the milk at elevated temperature, normally between 135° and150°C. for between 4 and 15 seconds. As a result of the heat treatment,microorganisms are destroyed so that the resultant product is a milk oflong shelf-life at room temperature. However, the drawback inherent inmilk treated by the UHT method is that the high temperature imparts tothe milk ac cooked flavour.

With a view to reducing the contents of harmful microorganisms, use hasrecently been made of microfiltration, a method which is described inSwedish Patent Specification SE 451 791. In the described method, use ismade of a microfilter, normally of ceramics, through which the skimmedmilk fraction, after separation, is caused to pass. In the microfilter,the skimmed milk is divided into a permeate flow which has aconsiderably reduced content of microorganisms, and a retentate flow,which contains the majority of bacteria and spores. The retentate istreated at elevated temperature and thereafter returned to the permeate.This method gives a product which has a longer shelf-life than normal,pasteurised milk, but it must be kept under cold storage and cannot beconsidered as sterile.

In Swedish patent application 9602546-5, use is made of microfiltrationto produce an aseptic consumer milk. By employing a microfilter ofslight pore size, milk with a higher fat content can be filtered. Thepermeate flow, which contains the major fraction of the thermosensitivewhey proteins, is free of microorganisms. The retentate flow, whichcontains fat, casein and microorganisms, undergoes a high temperaturetreatment, whereafter both of the flows are mixed. Employing thismethod, there will be obtained a commercially sterile product which,however, contains the destroyed microorganisms.

The present invention relates to a method of obtaining, primarily, acommercially sterile skimmed milk for consumption, but by the admixtureof the cream fraction in suitable proportions, it is possible, employingthis method, to obtain a consumer milk with varying fat content. Thepresent invention also entails a method which gives a sterile productwhich is as good as free of destroyed microorganisms and which therebymay be considered as purer than milk produced according to prior artmethods.

OBJECTS OF THE INVENTION

One object of the present invention is to obtain a commercially sterileskimmed milk intended for consumption, which is of extreme purity inthat the majority of the destroyed microorganisms are not contained inthe finished product.

A further object of the present invention is to realise a sterile milkproduct with improved flavour properties, since the method involves heattreatment at relatively low temperatures.

SOLUTION

These and other objects have been attained according to the presentinvention in that the method of the type disclosed by way ofintroduction has been given the characterizing features that the skimmedmilk is treated by microfiltration for obtaining a retentate flow and apermeate flow, and that the permeate flow is thereafter heat treated ata temperature of between 72° and 134°C.

Preferred embodiments of the present invention have further been giventhe characterizing features as set forth in the appended subclaims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

One preferred embodiment of the present invention will now be describedin greater detail hereinbelow, with reference to the accompanyingDrawing, in which:

FIG. 1 is a flow diagram illustrating a facility for reducing the methodaccording to the present invention into practice.

DESCRIPTION OF PREFERRED EMBODIMENT

The raw milk which enters the dairy through a conduit 1 is separated ata temperature of 4-60°C. A common temperature in separation in aconventional separator 2 is 55°C. In the separator 2, the milk isdivided up into a cream fraction and a skimmed milk fraction. Theskimmed milk has a fat content of approx. 0.05-0.15%. The cream fractionis treated separately, as will be described in greater detail below, andthe present invention relates essentially to the skimmed milk fraction.

In a conduit 3, the skimmed milk is led from the separator 2 to a firstmicrofilter 4. The microfiltration takes place most effectively if themilk is at a temperature of approx. 50°C. The microfilter 4 may, forexample, be made of ceramics. In the microfilter 4, the skimmed milkfraction is divided into a permeate flow and a retentate flow.

In a first embodiment, the microfilter 4 has an effective pore size of0.5 μm, i.e. the filter 4 should be capable of effectively separatingfrom the permeate flow all microorganisms which are larger than or equalto 0.5μm, including all spore normally occurring in the milk.Microorganisms are defined as all bacterial and spores normallyoccurring in the milk. The size of the microorganisms is defined bytheir least diameter, and effective separation is taken here to signifya reduction of the number of microorganisms by a factor of ≳1,000,000(≳log 6 reduction).

Trials have shown that a microfilter 4 according to this firstembodiment of the invention, with an effective pore size which is≳0.5μm, should preferably be combined with a subsequent heat treatmentat a temperature of 90-105°C. By a combination of these two methodsteps, there will be obtained a product, a skimmed milk, which iscommercially sterile, i.e. the product is free of microorganisms whichcan grow under the prevailing conditions.

In a second embodiment of the present invention, use is made of amicrofilter 4 with an effective pore size which is ≳0.3 μm, whicheffectively separates the microorganisms which are ≳0.3 μm, includingthe major proportion of thermoresistant bacteria and all spores. In thissecond embodiment, the microfiltration is combined with a heat treatmentat 72-98° C. and this combination of method steps realises acommercially sterile product.

Depending upon filter type, how the filter 4 is constructed or how it isdesigned, the microfiltration may take place in one or more steps. Thenumber of steps may depend on the size of the total quantity of filteredproduct which is made up by the retentate flow. The number of steps mayalso depend upon whether the intention is to employ the retentate flowor not, and on that efficiency which is obtained with a filter 4.

In those trials which have been carried out with a microfilter 4displaying an effective pore size of 0.5 μm, the permeate flowconstitutes 95% of the skimmed milk fraction. The retentate flow, whichconsequently constitutes 5% of the skimmed milk fraction, contains allseparated microorganisms which are ≳0.5 μm.

With a view to obtaining a greater yield from the method according tothe present invention, the retentate flow is led, in the preferredembodiment, from the first microfilter 4 into a conduit 5 to a secondmicrofilter 6. The second microfilter 6 may, for example, be made ofceramics and it should have at least the same effective pore size as thefirst filter 4. In the second filter 6, the incoming portion (5%) of theskimmed milk fraction is divided into a new permeate and retentate flow.The permeate flow from the second filter 6 consists, in the example, of4% of the incoming skimmed milk fraction.

The retentate flow from the second filter 6, which constitutes less than1% of the total skimmed milk fraction, thus contains all of themicroorganisms separated from both of the filters 4, 6 which are ≳0.5μm. The retentate flow from the second filter 6 is led off and, in thepreferred embodiment of the present invention, not employed in theproduction of a sterile skimmed milk according to the invention.

The method may also include additional filters 4, 6 which cooperatecorrespondingly, so that the retentate from the second filter 6 is ledto a third filter, and so on. But since the retentate flow already fromthe second filter 6 constitutes less than 1% of the total skimmed milkfraction, additional filters 4, 6 will not increase the yield from themethod to any appreciable degree.

The permeate flows from the two microfilters 4, 6 are combined in aconduit 7 and led to some form of heat treatment equipment 8 which mayconsist of a conventional plate heat exchanger. The two combinedpermeate flows thus contain no microorganisms which are larger than 0.5μm in the first embodiment and no microorganisms which are larger than0.3 μm in the second embodiment. Microorganisms remaining in the milkrequire a moderate temperature in order to be neutralised. The twopermeate flows are heat treated in the heat treatment equipment 8 at72-134° C., preferably at 90-105° C. in the first embodiment and at72-98° C. in the second embodiment for a requisite period of time.

The heat treated product which consists of more than 99% of the skimmedmilk fraction is now commercially sterile and is led in the conduit 9further to packing in an aseptic filling machine 10, where the productis packed in aseptic packages which may be distributed and stored atroom temperature. The finished product is also extremely pure, since itcontains no other destroyed microorganisms than those neutralised in theheat treatment and these constitute a infinitesimal quantity of themicroorganisms which initially exist in untreated milk.

The cream fraction is treated separately and, after separation, it isled in the conduit 11 via standardisation equipment 12 to a hightemperature treatment unit 13, the unit 13 normally also including ahomogenizer. In fat contents >12% in the cream fraction, a part of thesterile skimmed milk may be employed in the homogenization.

By selecting, in the standardisation equipment 12, a certain proportionof cream for admixture into the skimmed milk fraction it is possible toobtain a consumer milk displaying a given, desired fat content. Thestandardised, sterile product is packed aseptically in accordance withthe foregoing for obtaining a commercially sterile, standardisedconsumer milk.

As will have been apparent from the foregoing description, the presentinvention realises a method of producing a commercially sterile skimmedmilk intended as consumer milk, the skimmed milk being extremely puresince it does not contain the quantity of destroyed microorganisms whichsterile milk normally contains. Given that the milk is exposed to atemperature treatment which is considerably lower than conventional UHTmilk, it possesses improved flavour properties, and the flavour may becompared with the flavour of pasteurised milk.

What is claimed is:
 1. Filter apparatus for skimmed milk productioncomprising a first microfilter and a second microfilter each having aneffective pore size of about 0.5 μm or less, said filter apparatusincluding a supply conduit for supplying a stream of skimmed milk to thefirst microfilter, a first discharge conduit for the milk permeate, anda second discharge conduit for conducting the retentate from the firstmicrofilter to the second microfilter, a third discharge conduit forconducting the permeate from the second microfilter to the firstdischarge conduit where the respective permeate are mixed together, and,wherein said skimmed milk has a lower concentration of microorganisms bya factor of log 6 than said milk permeate.
 2. The filter apparatus ofclaim 1, wherein the milk permeate is free from all spores.
 3. Thefilter apparatus of claim 1, wherein the milk permeate has a reducednumber of thermoresistant bacteria compared to the skimmed milk.
 4. Thefilter apparatus of claim 1, having an effective pore size of about 0.3μm or less.
 5. The filter apparatus of claim 4, wherein the milkpermeate has a reduced number of thermoresistant bacteria compared tothe skimmed milk, and the milk permeate is free from all spores.
 6. Amethod of filtering skimmed milk comprising: passing skimmed milkthrough a first filter having an effective pore size of about 0.5 μm orless to form a first permeate flow and a first retentate flow,conducting the first retentate flow to a second filter having aneffective pore size of about 0.5 μm or less to form a second permeateflow and a second retentate flow, and combining the first permeate flowwith the second permeate flow, and subjecting the combined permeateflows to further processing, whereby this method reduces the number ofmicroorganisms in the combined permeate flow to less than the number ofmicroorganisms in said skimmed milk by a factor of at least log
 6. 7.The method of claim 6, wherein the combined permeate flow is free ofspores.
 8. The method of claim 6, wherein the combined permeate flow hasa reduced number of thermoresistant bacteria compared to the skimmedmilk.
 9. The method of claim 6, further including passing the retentateflow through a second filter having an effective pore size of about 0.3μm or less.
 10. A method for removing microorganisms from milkcomprising: passing skimmed milk through a first filter having a poresize of about 0.5 μm or less to form a first permeate flow and a firstretentate flow, passing the retentate flow from the first filter to asecond filter having a pore size of about 0.5 μm or less to form asecond permeate flow and a second retentate flow, and mixing the firstpermeate flow with the second permeate flow, whereby reducing the numberof organisms in the skimmed milk after the mixing step by a factor of atleast log
 6. 11. The method of claim 10, including subjecting the milkafter the mixing step to heat treatment at about 72 to 134° C.
 12. Themethod of claim 11, wherein the first and second filters have aneffective pore size of about 0.3 μm and the heat treatment is carriedout at about 72 to 98° C.
 13. The method of claim 10, including passingmilk through a separator to provide a supply of skimmed milk and cream,and causing the skimmed milk to pass to the first filter.
 14. Filterapparatus for skimmed milk production comprising: a first filter havinga pore size of about 0.5 μm or less, a second filter having a pore sizeof about 0.5 μm or less, a first conduit for conducting retentate fromthe first filter to the second filter, a second conduit for conductingpremeate from the first filter to a predetermined location, and a thirdconduit for conducting the premeate from the second filter to thepredetermined location, whereby the microorganisms having a size that isequal to or greater than 0.5 μm are separated from the skimmed milk toprovide a product in which the microorganisms are reduced by a factor ofat least log
 6. 15. The filter apparatus of claim 14, including a heattreatment chamber and a third conduit for conducting the skimmed milkfrom the predetermined location though the heat treatment chamber. 16.The filter apparatus of claim 14, including a separator for separatingraw milk into a skimmed milk stream and a cream stream, and a thirdconduit for conducting the skimmed milk stream to the first filter.